External cardiac pacing utilizes relatively high and therefore potentially painful DC current (10-100 mA). This is required to achieve effective stimulation of cardiac muscle located deep within the chest. To deal with these and other problems, various external stimulating electrodes have been previously proposed for medical purposes. For example, U.S. Pat. No. 4,349,030 describes an electrode used for external cardiac pacing which includes as a skin contacting electrically conducting member a porous sponge formed from cellulose in which is absorbed an electrically conductive fluid. Electrical conductivity of the fluid provides a uniform current density to reduce stimulation of local sensory nerves. For example, a 1/4 inch thick cellulose sponge or a piece of gauze is dampened prior to use with tap water or a weak electrolyte. A fluid-like gel can also be used to saturate the sponge or gauze. In order to be absorbed into the fibrous material the gel must have fluid consistency. The sponge is connected to an electrically conductive metal backing having an insulated wire permanently attached to it. In commercial equipment utilizing the patented structure, a male plug is connected to the free end of the insulated wire. During use, the male plug connects to a female receptor located in the external cardiac pacing device or power supply. Other commercially available external cardiac pacing electrodes used in conjunction with specified external cardiac electrical pacing devices are also manufactured with permanent wiring having a wire for supplying current with a unique male plug at its free end that is adapted to connect to a complementary female receptor located in the external pacing equipment.
These prior electrodes suffer from several important shortcomings. First, they can only be used with specific electrical pacing equipment because the male plugs wired in place permanently will only fit a specific piece of equipment. Second, the construction of the electrode makes it relatively expensive because of its size and bulk. Moreover, the electrolyte applied to the sponge must necessarily be in a fluid state to flow into the pores of the sponge or gauze supporting it. Its fluidity however allows the electrolyte to smear, run or leave residue on the patient's body, as well as drying out on the shelf.
U.S. Pat. No. 4,274,420 describes a monitoring and stimulation electrode. The broad statement is made that the resistivity can be anything less than 10,000 ohms-meter (i.e. less than 1,000,000 ohms-centimeter), but the preferred resistivity is less than 1,000 ohms-meter or 100,000 ohms-centimeter. While the patent does disclose a broad range for the purpose of showing that the resistivity is not unduly restrictive, it does not instruct or direct one to a useful level of resistivity employed in accordance with the present invention. Throughout the patent, it is emphasized that there is an advantage in using low AC resistivity in electrodes, e.g. column 3, lines 20-26; column 4, lines 31-32; and example 20, lines 15-20. This is consistent with teachings elsewhere in the patent which suggest the use of relatively large quantities of strongly ionized electrolytes such as sodium chloride. Thus, although the suggestion is made that the electrode can be used for stimulation purposes, there is no disclosure of how one can achieve a painless heart stimulation using constant current, i.e. DC current.
Other stimulating electrodes used for Transcutaneous Electrical Nerve Stimulation (TENS) and neuromuscular stimulation (NMS) utilize a conductive adhesive and a conductive rubber backing. TENS typically employs AC electricity at about 10 mAmp current levels. Because of the alternating nature of this current it generally is considered less painful and irritating than comparable DC levels. One design application of these electrodes, NMS, typically employs DC electricity at about 10 mAmp current levels. This is more similar to cardiac pacing than TENS yet operates at much lower current levels and therefore has much less potential for pain generation than cardiac pacing. Ordinary electrodes, however, do not provide optimal results for painless cardiac stimulation, in part because the electrically conductive matrix will allow current to concentrate in areas of lower impedance occurring in the skin surface. This can occur progressively as the patient is stimulated, causing a runaway condition in which the skin of the patient may even break down electrically, allowing a substantial surge of current to flow through a localized area, causing extreme pain to the patient. Moreover, although conventional TENS and NMS electrodes may provide comfortable stimulation in their designed purpose, both of these electrodes are single purpose electrodes unsuited for cardiac pacing applications due to their relatively high conductivity.
______________________________________ Typical Applied Current Levels for Various Stimulation Applications ______________________________________ Transcutaneous Nerve Stimulation 1-10 mA (AC) External Cardiac Pacing 10-60 mA (DC) Neuro-Muscular Stimulation (NMS) 1-40 mA (DC) ______________________________________
In view of these and other deficiencies of the prior art, it is a general object of the invention to provide a multipurpose medical electrode suited for Transcutaneous Electrical Nerve Stimulation, neuromuscular stimulation and for cardiac pacing with a provision for reliably preventing current build-up in localized areas and a runaway condition in which a surge of current passes through a localized area of the skin where electrical resistance is broken down under certain applied current loads. A further object is to provide an electrode wherein the electrically conductive skin-contacting matrix is a stable, semi-solid gel that will not run, smear, migrate or flow out onto the patient's body, requires no porous cellulose structure for support, is substantially homogeneous throughout, has a tacky surface adapted to make excellent electrical contact with the skin and to conform to surface irregularities and to flex with the movement of the body and can be applied as a coating layer onto sheet material.
These and other more detailed and specific objects of the invention will become apparent in view of the following specification and drawings which illustrate by way of example but a few of the various ways in which the present invention can be accomplished within the scope of the appended claims.